The art for the reduction of NOx from lean burn combustion sources has found the injection of ammonia or urea into exhaust ducts and flues upstream of a selective catalytic reduction (SCR) catalyst to be a solution to the issue of NOx emissions from diesel engines, gas turbines, boilers and the like. Urea, while a preferred reagent for SCR due to its safety and handling characteristics compared to ammonia, generally requires complicated and costly urea decomposition schemes directed at converting the aqueous urea reagent into ammonia gas prior to injection of ammonia gas into the exhaust through a series of ammonia distribution pipes or an ammonia injection grid (AIG). The AIG is located upstream of the SCR catalyst and positioned horizontally or vertically across the inside of an exhaust gas duct.
The direct injection of aqueous urea through a grid has generally not been practicable due to the formation of deposits in the grid from the incomplete decomposition of urea in the grid. This has led to efforts to inject urea at the wall of the duct, which while successful on small SCR applications, such as passenger car diesel exhausts and even larger stationary engines, has not generally been applied to the larger duct sizes and gas flows found on industrial boilers, furnaces and combustion turbines. That is generally due to: the inability to achieve sufficient penetration and distribution of urea reagent injected from the wall into the full exhaust gas flow; the inability to complete the process of urea decomposition to ammonia in the exhaust gas before the SCR catalyst; and, the inability to establish uniform distribution of ammonia at the SCR catalyst face, especially with short residence times or in large cross sectional ducts, or in ducts with complex physical arrangements, such as turns and bends or when operating at very low injection rates such as at low combustor loads.
The prior art discloses the wall injection of aqueous urea and ammonia reagents on large boilers using the selective non-catalytic reduction process (SNCR). In the case of SNCR, the reagent is generally mixed with dilution water and injected as a very dilute solution with high volumes of liquid in the high temperature zone of the furnace at temperatures of 1700° F. to 2100° F. The dilution water ensures penetration of the reagent across the furnace. For example, in U.S. Pat. No. 3,900,554, Lyon discloses the injection of an aqueous solution of ammonia into a furnace at a true temperature of 1300° F. to 2000° F. for the selective non-catalytic reduction of NOx. In U.S. Pat. No. 4,842,834, Burton describes a process for injecting reagents, including urea, into large furnaces at a temperature of 1700° F. to 2500° F. using an atomization conduit positioned on the furnace wall. Peter-Hoblyn et al., in U.S. Pat. No. 5,342,592, describes the use of retractable and water-cooled injectors for the injection of reagent across large furnace dimensions with high flue gas temperatures.
However, these prior art systems are generally not practicable in connection with employing relatively low volumes of concentrated reagent for small boilers or gas fired combustion systems using SCR. The low volume of reagent makes control of injection rate and the distribution of the reagent in the gas stream difficult, especially at low combustor loads.
In the SCR process, the distribution of the reagent across the duct and the mixing of the injected reagent with the exhaust gases containing the NOx is critical to having a uniform distribution of reagent and exhaust gases at the face of the catalyst in order to maximize NOx reduction and to avoid un-reacted reagent from slipping by the catalyst.
CFD modeling can be used to simulate the dimensions of a duct and the gas flow through a duct, and CFD techniques can be used to identify preferred injector locations to minimize the mal-distribution of reagent across the catalyst face as described in co-pending U.S. Patent Application Publication No. US 2014/0099247 A1 to Jangiti et al., also assigned to assignee of the present application. U.S. Patent Application Publication No. US 2014/0099247 A1 is hereby incorporated by reference herein in its entirety.
In U.S. Patent Application Publication No. 2014/0099247, Jangiti et al. describes a method for using computer modeling to identify the optimum location for installing wall mounted injectors on the exhaust duct of a lean NOx combustor to improve the distribution of reagent at the downstream catalyst. While helping to optimize injection location, such that the RMS at the catalyst is generally less than 15%, Jangiti et al. does not specifically explore the mechanical limitations on operating certain types of injectors at low injection flow rates such as required at low boiler loads, nor how to maintain good reagent distribution at a downstream catalyst where the traditional method would be to take certain injectors out of service at low flow conditions.
Pulse width solenoid actuated injectors with known spray characteristics, such as those described in U.S. Pat. No. 5,976,475 to Peter-Hoblyn et al. and U.S. Pat. No. 7,467,749 to Tarabulski et al., represent one method to accurately introduce reagent into the duct. Such injectors can be mounted directly on the exhaust duct wall or can be mounted to an air lance as is disclosed in co-pending U.S. Patent Application Publication No. US 2012/0177553 A1 to Lindemann et al., also assigned to assignee of the present application (hereby incorporated by reference herein in its entirety). In many cases, especially on larger ducts, it is preferred to use multiple injectors around the periphery of the exhaust duct in locations determined in part by CFD modeling. This provides better distribution of the reagent across the duct versus a single point of injection.
An advantage of the pulse width solenoid actuated injector is that the injection rate can be varied instantaneously at a frequency of 5 or 10 Hz (5 or 10 injection events per second) by holding the valve in an open position electronically for some programmable period (“injector on-time”). This can be done by a computer that has the ability to adjust the injector on-time automatically based on a process signal such as boiler load, exhaust gas flow rate, exhaust gas temperature or a NOx signal. Typical injector on-times range from 10% to 85%. Outside of this range the injector can lose accuracy between pulses.
Multiple injectors can also allow for better turndown in the reagent injection rate to respond to lower boiler loads requiring lesser amounts of reagent. One method of turndown is to take certain injectors out of service at lower loads and use only a few remaining injectors. However this causes an imbalance in the location of the reagent injected relative to the bulk exhaust gas flow in the duct and can lead to lower NOx reductions by depriving some of the exhaust gases of reagent for reaction across the catalyst.
In U.S. Pat. No. 8,109,077, Reba et al. describes a method for injecting a fluid, such as aqueous urea, into the exhaust of a diesel engine and describes a dual injector system generally directed at injecting two different reagents. In other cases where the reagent is the same, one injector can be on while the other is off depending on engine operating conditions. While the concept of having one injector on and one injector off is described by Reba et al., there is no mention of sequencing of the injectors in an on/off pattern to achieve good distribution in a duct across a downstream catalyst. In the application of multiple injectors to a diesel engine exhaust as described by Reba et al., the duct diameter is generally small and the velocity is high thereby making the mixing and distribution of reagent across a downstream catalyst less of a concern than in large dimension exhausts ducts with low velocity and generally poor mixing.
Reba et al. speaks to one of the issues addressed by the present invention by describing the variation in flow from a solenoid actuated return flow injector of the preferred type as +/−20% when operated at low flow rates and Reba et al. describes the desire to operate above 10% on time as much as possible. The current invention recognizes the issue with solenoid actuated injectors at low flow and teaches the novel concept of shutting injectors off and on in a sequential pattern at low flow such that the need to operate multiple injectors concurrently at low on times is reduced by sequentially operating only one injector at a time, or in the case of pairs of injectors, only one pair, at a time.
Reba et al. discloses the use of a lead injector and a lag injector and operating injectors at different frequencies and on times to cover a broad range of injection ranges. However there is no disclosure of alternately sequencing the injectors for short bursts of injection at low loads to accommodate very low injection rates while maintaining good distribution of reagent across a downstream catalyst, as does the current invention.
In U.S. Patent Application Publication No. US 2003/0109047 A1, Valentine teaches a plurality of separately controlled injectors incorporating a metering valve and air lance for the introduction of pollution control chemicals into the effluent of a combustor over widely variable combustor loads. The approach of Valentine is directed at SNCR applications where large volumes of reagent are injected into the furnace at high temperatures for a gas phase reaction with NOx local to the point of injection. While Valentine teaches the ability to adjust injection rate, Valentine does not recognize the inherent issue with solenoid actuated injectors related to their loss of accuracy at low injection rates and so does not suggest the concept of sequencing the operation of individual injectors between off and on when operating at low loads.
In fact, in the application of Valentine for SNCR, the use of sequential injection would not be beneficial as the reagent would be injected in high concentrations at the point of injection and would not have the opportunity to mix out across the furnace in the preferred temperature zone for the SNCR NOx reduction reaction. In contrast, the current invention is directed at sequential injection in an SCR process where given sufficient mixing and residence time the concentration of reagent from sequential on/off operation of injectors can still be distributed and mixed with the exhaust gases before reaching the face of the downstream catalyst.
Thus, the problem remains in accordance with known prior art designs, that at low boiler loads the total reagent injection rate can be so low that multiple injectors cannot simultaneously be operated to inject at such a low quantity (and low % on-time). Moreover, in accordance with known prior art designs, taking some injectors out of service to maintain a minimum reagent flow through the remaining injectors creates mal-distribution of reagent and localized high spots of reagent concentration in the duct and at the catalyst face.